By Ava Veith; David McCall, Ph.D.; Daniel Sandor, Ph.D.; Kyley Dickson, Ph.D.; Travis Roberson; Aaron Tucker; and Conlan Burbrink
[Editor’s note: This article is written specifically from the lead author’s (Ava Veith’s) perspective.]Turfgrass diseases can cause many sleepless nights for field managers. Often, these diseases severely disturb the aesthetic and natural beauty of a healthy turfgrass athletic field. They come in a variety of shapes and sizes, and each patch, spot, or ring that appears within an area of turfgrass is surely a distraction to the eyes of viewers. But are turfgrass diseases more than simply a cosmetic issue on athletic playing surfaces?
During the summer of 2023, several researchers from Virginia Tech and the University of Tennessee studied the impacts of spring dead spot and winterkill on field playability as it relates to athlete safety. Athlete safety is a topic that hits close to home for me as a four-year member of the Virginia Tech women’s soccer team. I love being able to connect my passion for athletics with the research I am doing.
Spring dead spot is one of the most common diseases of bermudagrass in regions where the grass experiences winter dormancy, such as throughout the transition zone . The fungal pathogen Ophiosphaerella infects the turfgrass in the fall while soil temperatures are between 50°F and 75°F; however, symptoms are not present until the spring after green-up occurs (hence the name). The disease appears as a patch of dead turfgrass that varies in size. It creates a sunken surface in the turfgrass canopy, leading to significant variability in field uniformity. Winterkill is not technically a disease; it is caused by severe temperature fluctuations during winter months, leaving large areas of completely dead turfgrass following spring green-up. Both spring dead spot and winterkill are common on athletic fields, and their presence creates an obvious uneven surface within the turfgrass canopy.
Our research objective was to quantify the effects of spring dead spot and winterkill on field performance and athlete safety using various metrics. We hypothesized that spring dead spot and winterkill will create adverse playing conditions, such as increased surface hardness and decreased foot traction, causing athlete safety concerns on these inconsistent surfaces.
Research methodology
Spring dead spot and winterkill were present on several athletic fields throughout southwestern Virginia (and many other transition zone locations) during the spring and summer of 2023. Data were collected at four locations: two fields in Blacksburg and two in Roanoke. The fields in Roanoke were of particular interest because they were used by my summer league team at the time, and my teammates and I could physically feel the differences between areas of asymptomatic turfgrass versus areas with spring dead spot and winterkill while practicing.
A 2.25 kg Clegg hammer was used to measure surface hardness, a FieldScout TDR 350 to measure soil moisture, a Turf-Tec Shear Strength Tester (shear vane) to measure the rotational shear strength of the turfgrass, a ball rebound device to record the bounce height of a FIFA-grade soccer ball when released from two meters, and a FLEX testing device to simulate an athlete’s stopping or accelerating motion. The shear vane measures the rotational resistance of the turfgrass by simulating an athlete’s cleat planting and changing direction. It measures the force (Nm but converted to inch-pounds for this article) required to ‘shear’ the surface. The shear vane used was about 80 pounds. The FLEX device was created by University of Tennessee’s professor John Sorochan, Ph.D. and Kyley Dickson, Ph.D. It is portable and was designed to simulate the foot strike of an athlete when stopping or accelerating. It takes four separate measurements in one strike:
1. Vertical force: hardness of the surface experienced at the ankle.
2. Rebound: energy restitution of the surface or recoil.
3. Displacement: stability of the surface.
4. Horizontal force: horizontal traction force.
At each field location, data were collected from 20 spring dead spot patches and 20 areas with winterkill and compared against adjacent asymptomatic matched pairs. Additionally, 10 of the 20 spring dead spot patches and asymptomatic areas tested were irrigated with 0.25 inches of water to see if an increase in soil moisture from potential rainfall or planned irrigation would worsen the effects of the disease. The decision to irrigate was made due to results from a previous study, in which the impacts of spring dead spot were more drastic on a field with higher soil moisture than on nearly fields with lower soil moisture.
Data were analyzed using JMP 16 (version, SAS institute, Cary, N.C.), where means were separated using the Each pair Student’s t-test when appropriate (α=0.05).
Results
Surface Hardness
Overall, spring dead spot did not impact surface hardness before or after irrigation in our locations. Gmax values ranged from 75.8 inside of spring dead spots to 73.5 in asymptomatic regions prior to irrigation, and from 66.6 to 69.5 in the same areas immediately following irrigation. Both spring dead spot and asymptomatic turfgrass were within the 60-80 Gmax range for acceptable playing conditions, whether irrigated or not. As expected, irrigation caused Gmax values for both spring dead spot and asymptomatic areas to decrease. However, winterkilled areas did impact surface hardness (p= <0.0001*). Winterkill creates a substantially harder surface than asymptomatic turfgrass, as the average value for winterkill areas was over 100 Gmax (Figure 1). This presents a concern regarding athlete safety, as the risk of concussion is increased if an athlete were to fall on a winterkill area during competition. Additionally, the risk of other acute injuries, such as stress fractures, and overuse injuries such as shin splints is increased when constantly running on a hard surface.
Rotational shear strength
Results suggest that spring dead spot patches and winterkill areas both have significantly lower rotational shear strength than asymptomatic turfgrass. Before irrigation, spring dead spot patches had an average shear strength of 415 inches-pounds, while asymptomatic areas had an average shear strength of 497.4 inches-pounds (p= 0.0001*). After irrigation, spring dead spot patches had an average shear strength of 420.4 inches-pounds, while asymptomatic areas had an average shear strength of 457.5 inches-pounds (p= <0.0001*). Thus, irrigation caused the rotational resistance of asymptomatic areas to decline significantly when compared to non-irrigated asymptomatic areas (p= 0.0004*). For winterkill areas, the average shear strength was 426.6 inches-pounds, while asymptomatic areas had an average shear strength of 518.6 inches-pounds (p= 0.0001*). This presents a concern for athlete safety, as athletes have less traction and are more likely to slip if planting their foot in a spring dead spot patch or an area with winterkill.
Ball rebound
Both spring dead spot patches and areas with winterkill had a significantly higher ball bounce than asymptomatic turfgrass as shown in figures 2 and 3 (p= <0.0001*). Additionally, significant differences were found in ball bounce height between asymptomatic turf and spring dead spot patches for both irrigated and non-irrigated areas (p=<0.0001*). Symptomatic turfgrass lacks a lush turfgrass canopy to cushion the impact of the ball, which may have led to this result. Speaking from experience, misjudging the ball trajectory after bouncing from the turfgrass surface during play negatively impacts an athlete’s performance.
Results from FLEX testing device
Surface stability (displacement) and recoil (rebound) on the ankle were significantly impacted by spring dead spot (p<.0001). Non-irrigated spring dead spot patches had an average rebound of 4.8 pounds of force while non-irrigated asymptomatic areas had an average rebound of 4.2 pounds of force. Irrigated spring dead spot patches had an average rebound of 4.5 pounds of force, while irrigated asymptomatic areas had an average rebound of 4.0 pounds of force. Spring dead spot patches had a higher rebound than asymptomatic areas whether irrigated or not, meaning more energy is being returned back to the athlete or ball upon contact with the playing surface. Some of the force from an athlete running, jumping, or falling is absorbed into the ground upon contact; however, when contacting a spring dead spot patch, more of that energy is being returned back in the athlete’s body. This places additional stress on ligaments, joints, and bones, which can lead to injury. On asymptomatic turfgrass, more of this energy is absorbed, which reduces stress on the body. This result is supported by the ball rebound data, as the bounce height when a ball was dropped on a diseased patch was significantly higher than on asymptomatic turfgrass.
The displacement, or stability, of the surface was significantly higher for spring dead spot patches than asymptomatic turfgrass. Non-irrigated spring dead spot patches had an average displacement of 2.9 inches, while non-irrigated asymptomatic areas had an average displacement of 2.6 inches. Irrigated spring dead spot patches had an average displacement of 3.0 inches, while irrigated asymptomatic areas had an average displacement of 2.7 inches. When the cleat from the FLEX device was planted into the turfgrass to simulate an athlete’s stopping or accelerating motion, more of the diseased turfgrass was displaced than the asymptomatic turfgrass. Again, this is supported by our shear vane results, where less force was required to shear the diseased surface.
Winterkill impacted vertical force (p=.0129) but not other metrics (p>/=.XXX). The hardness of winterkill surfaces experienced at the athlete’s ankle was an average of 896.5 pounds, while the hardness of asymptomatic surfaces experienced at the athlete’s ankle was an average of 757.8 pounds. This result is supported by findings with the Clegg, where areas with winterkill were also significantly harder than asymptomatic areas.
Soil moisture impacts on spring dead spot
Data collected from half of the matched pairs occurred immediately after 0.25” of irrigation at each location. Surface hardness decreased by 14% in irrigated spring dead spot patches, dropping values from 75.8 Gmax without irrigation to 66.6 Gmax after irrigating. Similarly, irrigating spring dead spot patches decreased energy recoil and displacement by 7% (from 4.8 pounds of force before irrigation to 4.5 pounds of force after irrigation) and 3% (from 2.9 inches before irrigation to 3.0 inches after irrigation), respectively. Additionally, ball bounce also decreased by 10% inside of spring dead spot patches following irrigation (39.5 inches before irrigation to 35.9 inches after irrigation). These data suggest that soil moisture does impact the surface conditions inside of spring dead spot patches.
Conclusions
Overall, the data suggests the presence of spring dead spot and winterkill on athletic fields affect surface properties that have been shown to, in turn, affect athlete safety and field playability. Winterkill creates a harder surface, increasing the risk of athlete concussions and other impact injuries (Dickson et al., 2018, Twomey et al., 2012, Dixon et al., 1999). Both spring dead spot and winterkill reduce rotational resistance relative to asymptomatic turfgrass, increasing the risk of athlete slippage. Ball bounce was higher on symptomatic turfgrass than asymptomatic turfgrass, affecting the field’s playability as athlete anticipation of ball trajectory is altered. Spring dead spot patches return more energy to the athlete upon contact and have less structural stability than asymptomatic turfgrass due to reduced rooting (Tredway et al., 2009), which increases the likelihood of impact injuries and slipping.
These findings reveal that spring dead spot and winterkill are more than simply a cosmetic or aesthetic issue, and can influence the safety of athletes and ball-surface interaction during competition.
Ava Veith is a graduate research associate at Virginia Tech, and a former Division I soccer player at Virginia Tech; David McCall, Ph.D., is associate professor, Turfgrass Pathology and Precision Management, Virginia Tech; Daniel Sandor, Ph.D., is collegiate assistant professor, Turfgrass Science, Virginia Tech; Kyley Dickson, Ph.D., is associate director Center for Athletic Field Safety and researcher, University of Tennessee; Travis Roberson is graduate research associate, Virginia Tech; Aaron Tucker is graduate research associate, Virginia Tech; and Conlan Burbrink is graduate research associate, University of Tennessee.
References
Dickson, K., Strunk, W., and Sorochan, J. (2018, February). The effect of soil type and moisture content on head impacts on natural grass athletic fields. In Proceedings (Vol. 2, No. 6, p. 270). MDPI.
Dixon, S. J.; Batt, M. E.; and Collop, A. C. (1999). Artificial playing surfaces research: a review of medical, engineering and biomechanical aspects. International Journal of Sports Medicine, 20(04), 209-218.
Tredway, L. P.; Tomaso-Peterson, M.; Perry, H.; and Walker, N. R. (2009). Spring dead spot of bermudagrass: A challenge for researchers and turfgrass managers. Plant Health Progress, 10(1), 32.
Twomey, D. M.; Finch, C. F.; Lloyd, D. G.; Elliott, B. C.; and Doyle, T. L. (2012). Ground hardness and injury in community level Australian football. Journal of Science and Medicine in Sport, 15(4), 305-310.
